Bub, K. and R . F . Bowerman 1979 . Prey capture by the scorpion Hadrurus arizonensis Ewing (Scor-piones, Vaejovidae) . J . Arachnol ., 7 :243-253 .

PREY CAPTURE .BY THE SCORPIONhIADRURUS ARIZONENSIS

EWING (SCORPIONES : VAEJOVIDAE )

Kathleen Bub and Robert F. Bowerman

Department of Zoology and PhysiologyUniversity of Wyoming, Laramie, Wyoming 8207 1

ABSTRAC T

Prey capture behavior of the desert scorpion Hadrurus arizonensis Ewing has been studied in th elaboratory . Information about the behavior was obtained from analysis of ten 8mm movie films, an d50 direct observations of individual prey capture sequences . An ethogram was constructed whichdepicts the interrelationships between the discrete behavioral components ; e .g ., alert stance, graspsuccess, and sting . Most individuals also exhibit a pedipalpal sand thrust, which may function in acleaning capacity.

INTRODUCTIO N

Scorpions are best known for their dramatic stinging behavior, which can be associate deither with prey capture or with defense . Still, scorpion prey capture behavior has no tbeen studied to any appreciable extent . Many previously published descriptions have bee nanecdotal in nature (Lankester 1883, Pocock 1893, Fabre 1911, Smith 1927) . Severalmore recent articles do include brief descriptions of prey capture for various scorpio nspecies (Vachon 1953, Baerg 1961, Cloudsley-Thompson 1961, Williams 1963, Stahnk e1966) . Hadley and Williams (1968) maintained several species in the laboratory, an ddescribed the prey capture behavior of Paruroctonus mesaensis Stahnke as representative .Palka and Babu (1967) described the ballistic defensive movements of the scorpion Heter-ometrus spp .

Robinson and co-workers have studied the prey capture behavior of several tropica lweb-building spiders (Robinson 1969, Robinson and Mirick 1971, Robinson and Olazarr i1971, Robinson et al . 1971, Robinson and Robinson 1973, 1974) . Ethograms of th epredatory sequence were designed and intraspecific variations noted . Prey capture by aprimitive web-builder (Valerio 1974) and by jumping spiders (Forster 1977) have alsobeen studied .

This paper represents the results of a study similar to the above on the predatorybehavior of a species of desert scorpion, Hadrurus arizonensis Ewing . The prey capturesequence is depicted in an ethogram, and the behavioral components are described .

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METHOD S

Individuals of H. arizonensis were collected in August 1977 near Palo Verde ,California . The scorpions were maintained individually in 25 cm x 35 cm terraria, with 1 5cm deep sandy-soil substrate, which permitted them to burrow . The individuals were o funknown age, ranging in length from 5 to 10 cm (prosoma to aculeus), and weighing fro m0.89 to 7 .36 grams. In the holding room, fluorescent lights and two heat lamps were o nfourteen hours and off ten hours, daily . Temperatures ranged respectively between 20 °and 29° C. Water was provided weekly by misting the substrate and by saturating asponge, upon which the scorpions were seen to knead with their chelicerae, presumabll yextracting the moisture . Individuals were removed from their terraria for observation onl yafter onset of the dark period, when they were out of their burrows . The observation an dfilming chamber was semicircular, with a 50 cm diameter .

Nymphs and adults of the American cockroach, Periplaneta americana, and adults ofthe common house cricket, Acheta domesticus, were offered to the scorpions as prey .Orthoperan species have been identified as natural prey for H. arizonensis (Hadley andWilliams 1968) . Time between feedings varied from one week to one month .

Ten prey capture sequences were filmed with a Beaulieu 4008 Zm 11 Super 8m mmovie camera, at 24 frames per second and analyzed with a Kodak stop action projector .No overt differences were noted between the behavior when filmed under white light sand that observed under red light . The spectral sensitivities ofH. arizonensis photorecep-tors have not been described, so it is not known whether or not the red light can b eperceived .

The next stage of the study involved 50 prey capture sequences by fourtee nindividuals, viewed under two 40W red tungsten light bulbs . Each scorpion wastransferred with forceps to the observation chamber and after a thirty-minute acclimatio nperiod, the cockroach or cricket was introduced . Subsequent activities were recorded on acassette tape recorder for later transcription .

RESULTS AND DISCUSSIO N

The prey capture sequence, as observed and analyzed in the laboratory, is represente dby the composite ethogram (Fig . 1) . The terms used in the ethogram are defined a sfollows :Motile—locomotion within the chamber, prior to contact with the prey .Retracted—body in contact with the substrate, metasoma and appendages drawn in .Alert Stance—a posture in which the scorpion is supported above the substrate by th elegs, the pedipalps are extended anteriorly, with the movable fingers of the pedipalpalchelae and the pectines in contact with the substrate (Fig . 2) .Orientation—movement of the scorpion resulting in the anterior aspect being directe dtowards the prey .Grasp Attempt—an effort to obtain a hold on the prey with the pedipalpal chelae .Grasp Failure—the prey escapes after a grasp attempt, whether there is contact or not .Grasp Success—the scorpion obtains a firm hold on the prey with at least one pedipalp .Sting—a behavioral unit consisting of a forward sweep to the metasoma, telson contac twith the prey and subsequent probing movements, and aculeus penetration wit hpresumed venom injection .

BUB AND BOWERMAN—PREY CAPTURE BYHADRURUSARIZONENSIS

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Inactive—following a grasp success ; no visually detectable activity of chelicerae, pedipalp sor walking appendages .Manipulation—handling of the prey by the pedipalps and first pair of legs, includin gturning of the prey for head-first ingestion .Cheliceral Activity—protraction of one chelicera and retraction of the second, alternatin gwith retraction of the first and protraction of the second . The chelicerae are opene dduring protraction and closed during retraction .

MOTILEALERT

2

RETRACTED

ORIENTATIO N

GRASPATTEMPT

GRAS P

FAILUREGRASP

SUCCESS

STING

A

INACTIVEMANIPU -

LATIONCHELICERALACTIVITY

SAN DTHRUST

TRAVE L

INGESTIO N

Fig . 1 .—Ethogram of the prey capture sequence and its constituent behavioral units for th e

scorpion Hadrurus arizonensis . Individual terms are defined in the text . Behavior observed in thi s

study was seen to flow in the direction of the arrows . The box circumscribing five of the behaviora l

units indicates that any of these units can precede or follow any of the others .

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Sand Thrust—a pedipalp is pushed into the substrate, withdrawn, and frequently brushe doff by the distal segments of the first and second pair of legs .Travel—moving throughout the chamber, holding the prey in a pedipalp and/or chelicerae .Ingestion—the intake of the pre-digested fluid prey, as indicated by cyclical movements o fthe coxae of the first legs .

The Prey Capture Sequence .—Many individuals, when first placed in the observatio nchamber, moved around the area in an exploratory fashion before settling . A potentialprey encountered during this motile stage, either was ignored by the scorpion, or induce dorientation and a grasp attempt . When a scorpion ceased to move around the chamber, i tusually adopted one of two postures . In some sequences (20%), the scorpions took on aretracted posture with the ventral surface resting on the substrate, and the pedipalps, leg sand metasoma drawn in against the body . In a few cases, scorpions in a retracted postur ecompletely ignored prey . At other times, however, the scorpion responded to nearby pre yby taking an alert stance or by making an immediate grasp attempt .

In most instances (80%), the scorpion adopted an alert stance (Fig . 2) soon after beingplaced in the observation chamber . In this posture, the prosoma and mesosoma aresupported slightly above the substrate by the four pairs of legs and the metasoma i scurled dorsomedially . The pedipalps are extended anteriorly, slightly bowed . The chelaeare open, with the very tips of the movable fingers in contact with the substrate . Thedistal aspects of the pectines are also touching the ground . Occasionally, scorpions wouldlift their pedipalps and pectines, reorient by moving forward or pivoting, then re-adop tthe alert stance . This activity often results in directed orientation of the scorpion toward snearby prey.

A scorpion in the alert stance was able to detect and orient to moving prey and t omake a grasp attempt before there was direct contact . Immobile prey did not elicit eithe rorientation or grasp attempts . On several occasions, a scorpion and prey would both stan dperfectly still within a few centimeters of each other for several minutes . Should the preymove first, the scorpion would lunge and often successfully grasp it, but when th escorpion moved first, the prey would usually avoid capture . The use of substrate-bornevibrations for prey detection by H. arizonensis seems quite possible . Brownell (1977 )demonstrated the ability of another vaejovid desert scorpion,Paruroctonus mesaensis, todetect compressional and surface waves generated by digging activity of prey up to adistance of 50 cm . Electrophysiological recordings showed that compound slit-sensilla o nthe basitarsal leg segments, and tarsal hairs responded to these prey-generated, substrat evibrations . Analogous sensory mechanisms may be , operating in H. arizonensis, possiblywith auxillary input from mechanoreceptors on the pedipalps and pectines, which ar eboth in contact with the substrate during the alert stance .

The alert stance of H. arizonensis brings legs, pedipalps, and pectines in contact withthe substrate simultaneously . The stereotyped placement of these appendages in thi sbehavior may be critical in permitting central nervous system integration o fenvironmental information concerning prey location . Another arthropod, the water strid-er Gerris remiges, utilizes a comparable stereotyped posture during prey orientating be-havior (Murphy 1971) . Localization of a stimulus and subsequent responses by thi sorganism are determined by which of the six legs are closest to the stimulus .

Even though the mechanical senses may be of highest importance, the prospect tha tthe visual system plays a role in prey location should not be discounted without directexperimentation . Recent studies (Fleissner 1977a,b), demonstrate the high sensitivity ofscorpion photoreceptors, particularly the lateral eyes .

BUB AND BOWERMAN—PREY CAPTURE BYHADRURUSARIZONENSIS

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Fig. 2 .—The alert stance of Hadrurus arizonensis, in which the distal aspect of the pectines and thetips of the movable fingers of the pedipalpal chelae are in contact with the substrate .

Fig. 3 .—Hadrurus arizonensis stinging an adult cockroach on the ventral surface . Note the involve-ment of both the mesosoma and metasoma in telson placement, and the supportive positioning of th elegs .

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In grasp success, both pedipalps are employed, with one or both gaining a firm hold o fthe prey . In 61% of the sequences, the first grasp attempt was successful, while in th eother 39%, two or more attempts were necessary . The attempt was considered a graspfailure if the prey was missed altogether, or was held only momentarily before escaping .Under natural conditions, should the scorpion fail in a grasp attempt, it is most likely thatthe prey, unless injured, would move from the vicinity and thereby reduce the prospectsfor a second attempt . In this study, where escape was not possible, the scorpions wereallowed unlimited grasp attempts .

The prey was stung at least once in all sequences observed . When stinging prey, H.arizonensis maintains postural stability by extending the second, third and fourth pairs o flegs in a characteristic pattern (Fig . 3) . The first pair of legs may be supportive as well ,but more frequently assists the pedipalps in the manipulation of the prey . The thrashinglegs of the victim may be held or moved aside by these anterior most legs . The pectine sremain inactive during stinging, being withdrawn against the ventral mesosomal surface .The telson is initially brought forward dorsomedially in a slow precise movement (Fig . 4) .The posture of the mesosoma during stinging ranges from a slight convex bowin g

E ~ 120-- =ens

0 .3 9sec

/---

Fig . 4 .-Progressive movements of the opisthosoma from point of grasp success to telson contact .

Tracings are from selected frames of an 8mm movie taken at 24 fps . Time periods between position s

are given, the entire movement taking approximately 0 .75 seconds .

BUB AND BOWERMAN—PREY CAPTURE BYHADRURUSARIZONENSIS

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involving every segment (Fig . 5A), to a more angular deflection (Fig . 5B). It is themetasoma, however, which demonstrates the greatest flexibility . As opposed to the moreor less uniplanar curving of the mesosoma, the first four joints of the metasoma have aconsiderable rotational ability (Bowerman 1972) . When the telson is brought forward, theaculeus first contacts that part of the prey's body in direct line with the sweep . If softtissue of the prey is met, penetration and presumably venom injection occur immediately .However, if hard chitinous plates are contacted first, the telson must then be repositione dto locate a penetrable tissue .

Some species of scorpion (e .g ., Euscorpius italicus and Anuroctonus phaiodactylus)seldom, if ever, employ a sting during prey capture (Schultz 1927, Cloudsley-Thompson1955a, Baerg 1961, Williams 1963, McDaniel 1968) . Baerg (1961) points out that scor-pions with large pedipalps and reduced metasoma probably do not use the sting fo rimmobilizing prey . In the current study, H. arizonensis stung every prey offered . It ispossible that H. arizonensis may not sting prey below a certain size, or under differentconditions, but these were not investigated .

The time from telson contact to actual penetration by the aculeus, i .e ., that time spentlocating soft tissue, ranged approximately from one to 50 seconds . In some cases, exten-sive telson movements occurred throughout this period .

Steiner's (1976) mapping of predatory digger wasp sting sites on cricket prey showe dclumped distribution, positively correlated with the location of major ganglia . Thereappears to be no such correlation in the sting sites of H. arizonensis on either cricket o rcockroach prey . Rather, the sting site distribution appears to reflect the sites of the firs tpenetrable tissue encountered . Adult cockroaches, for example, received 94% of th e

s

Fig . 5 .—Postures demonstrated by Hadrurus arizonensis when stinging prey. Drawings are fro m

single frames of 8mm movies .

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stings on their ventral surface, even though the dorsal surface was usually encountere dfirst by the telson . The heavy wings and thoracic sclerites appear to prevent dorsa lpenetration . Adult cricket prey, however, received 75% of their stings dorsally . The dorsa laspect was again usually encountered first, but since the relatively small cricket wings donot extend over the abdomen, immediate penetration could often occur .

Any one of the behavioral units following the sting, may follow any other as shown b ythe box and interconnections in the ethogram (Fig . 1) . For example, the initial sting maybe followed by inactivity, manipulation, cheliceral activity, sand thrusts or travel . Subse-quent stings occurred in 52% of the sequences, usually in response to struggling of th e

prey .Within this part of the prey capture sequence an interesting behavior termed the san d

thrust occurs which heretofore has not been described . The sand thrust (Fig . 6) wasobserved in 81% of the sequences and appears to be a fundamental part of prey capture i nthis species . Only one individual repeatedly failed to exhibit the sand thrust . Four otherscorpions failed to exhibit the sand thrust in one of several observed sequences . Theremaining nine scorpions invariably performed the sand thrust during the prey captur esequence .

Either simultaneously with, or following a sting, the scorpion releases one pedipalpa lchela from the prey while retaining hold with the other . The free pedipalp is thrust int othe sandy substrate up to the base of the chela . Usually, it is withdrawn immediately, bu tat other times it is kept buried for several seconds . Repetitive thrusts frequently occur ,and prior to regrasping the prey, the chela is often brushed off by the distal segments o fthe ipsilateral first and second legs . A sand thrust by one pedipalp is sometimes immedi-

Fig . 6 .-The sand thrust behavior demonstrated by Hadrurus arizonensis following the capture of

prey .

BUB AND BOWERMAN—PREY CAPTURE BY HADRURUSARIZONENSIS

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ately followed by a sand thrust with the other pedipalp . The function of the sand thrustbehavior is unknown . It may well be a grooming or cleaning behavior, as hemolymphfrom the prey often contacts the pedipalps during capture .

In the majority of the sequences (75%), the scorpions remained to feed at the sit ewhere the prey was captured . In the other sequences, they traveled throughout theobservation chamber with the prey held in one pedipalp or the chelicerae . This activitycontinued occasionally interspersed with digging behavior, until the scorpion eventuall ysettled to feed .

Cheliceral mastication began at the head end of the prey in 89% (39/44) of th esequences . In 30 of the sequences, this required that the prey be reoriented by manipula-tion with the pedipalps . In each of the other nine sequences, the prey was caught in suc ha way that the head was already directed toward the chelicerae . Other manipulationsinvolved displacement or removal of the prey ' s wings, leg and/or antennae . The head firstprey consuption seen in H. arizonensis has been described in Opisthophthalmus latimanusKoch by Alexander (1972), who identified the position of the prey's legs as being one o fthe clues used by the scorpion for feeding orientation . The possible advantages underlyingthis specific head-first ingestion were not discussed in her article . Head-first consumptio nmay assist in subduing the prey by immediately damaging the brain . Alternatively, thescorpion may be avoiding the posterior aspect of the prey, since many orthopterans havelarge powerful legs which could greatly interfere with the feeding activities of th escorpion . Similarly, chemical defenses in some prey such as tenebrionid bettles are ex-truded from the rear, and would be avoided by starting at the head .

The scorpion ' s first pair of walking legs aid the pedipalps in manipulating prey, whic his not an unusual characteristic among the arachnids. Within the Solifugae andAmblypygi, the first pair of walking legs are secondarily segmented for use as tactil eorgans, and are not used for locomation (Savory 1970). Certain Araneae are known to usetheir first pair of legs for display during courtship, as well as for walking . Bowerman(1975) in a study on the control of walking in H. arizonensis, noted that the first pair o flegs exhibited different stepping patterns than the more posterior legs, i .e ., they wer eelevated for longer periods during stepping . It was suggested that these legs functioned, i npart, in a tactile capacity . A closer investigation of these appendages in several scorpio nspecies would be of interest .

The chelicerae are activated prior to their actual contact with, and use in, masticatio nof the prey. The activity consists of reciprocating movements of left and rightappendages . One chelicera is protracted with open chela, at the same time that the othe rchelicera is retracted with closed chela . Then, each chelicera performs the motion jus tcompleted by the other . The reciprocate grasping and retraction by the two chelicera eslowly tears the prey's exoskeleton, exposing the inner tissues to the digestive enzymes o fthe scorpion (Snodgrass 1948) . Rhythmic movements of the coxae of the first pair of legs

help in transporting the predigested prey into the scorpion's oral cavity by means of apumping action (Shrivastava 1955) . Observations on the prey capture sequences were

terminated when the rhythmic movements of the coxae signaled that ingestion had com-menced .

Predation strategies of several species of desert scorpions have been observed in the

field . In general, the majority of these scorpions employ a sit and wait strategy (William s1963, Stahnke 1966, Hadley and Williams 1968, Enders 1975), ambushing prey tha t

blunders into the vicinity (Pianka 1973) . Some large diplocentrid species wait in theentrance of their burrows, while most of the vaejovid species move a distance from the

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burrow before settling . H arizonensis appears to utilize a sit and wait strategy involvingthe alert stance . The presumption that the prey capture sequence of H. arizonensis

unfolds similarly in the field to the way it does in the laboratory observation chambe rawaits verification . However, given that the just stated presumption is reasonably sound ,the current study provides an informational foundation for further studies on the feedin gbehavior of H. arizonensis as well as for comparative studies of other species of scorpion ,both closely and distantly related .

ACKNOWLEDGMENT S

The authors would like to express thanks to Oscar F . Francke, not only for guidancein locating effective collecting sites, but also for aid in subsequent species identification.Critical review of the manuscript by Michael H . Robinson was greatly appreciated .

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